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High fuel prices and concerns about climate change are boosting the popularity of gasoline/electric hybrid cars in the United States and abroad. However, environmentally minded motorists have another choice--diesels--which account for 45 percent of new passenger-car sales in European markets. Besides fuel economy concerns, the reasons include lower taxes on diesel fuel and greater tolerance of higher air emissions. Enthusiasm for diesels is relatively muted in the United States, with sales of approximately 30,000 diesels per year, mostly Volkswagens. Compare this to the sales of hybrid cars: more than 180,000 have been sold in the United States to date, 85,000 of them in the last year.

Americans generally think diesels are noisy, dirty, and underperforming. In a 2002 diesel vehicle survey by J.D. Power and Associates, 32 percent of Americans were concerned about engine noise, 27 percent about exhaust odor, and 31 percent about lower performance. However, these concerns are largely misplaced; most Americans remember the dirty diesels of decades past and are not aware of the advances incorporated into modern diesel vehicles. For example, in the last 15 years, diesel engines have reduced noise by 60 percent, emissions by 90 percent, and fuel consumption by 30 percent, while increasing torque by 100 percent. Modern diesels can be as desirable to consumers as gasoline automobiles, but how does their environmental performance measure up?

To answer this question, we must examine the entire lifecycle of the vehicle, including extracting raw materials, manufacturing and assembling automobile components, producing and combusting fuel, and maintaining and disposing of vehicles. Several differences between diesels and hybrids are undeniable: diesel engines are inherently more efficient than gasoline engines, diesel fuel contains approximately 10 percent more energy per volume than gasoline, and diesels produce significantly more air pollution. A closer look at the models offered today highlights these differences.

Three hybrid and three diesel passenger cars are currently offered in the United States (excluding the two-passenger Honda Insight and the luxury Mercedes-Benz E320 CDI). Table 1 provides basic size, weight, price, and performance data for these cars. The Honda Accord is the most expensive and the best performing; the other cars closely match each other in cost and performance. Overall, the diesel vehicles have a smaller cost differential than the hybrids.*

Materials Use and Waste Production

Nearly all vehicle energy consumption (more than 85 percent) and pollutant emissions occur during vehicle usage, which includes the vehicle emissions themselves as well as the impacts associated with the fuel cycle: the extraction, refining, transport, and use of diesel and gasoline. The large environmental impact during vehicle use means that materials use for manufacturing is not a primary concern, though it is worth some consideration. The vehicle lifecycle includes the production of about 6,700 kilograms (14,740 pounds) of hazardous waste per vehicle; half of that is produced during materials extraction and vehicle manufacture and half during fuel refining. Hybrid-car batteries are an additional source of hazardous waste, but the impacts are difficult to quantify. The potential longer lifetime of diesel vehicles also reduces materials use, though this may be offset by the extended usage of older vehicles with worse-than-average air emissions. Overall, there is no clear preference for either vehicle based on materials use.

Energy Use and Greenhouse Gases

The U.S. transportation sector produces 26 percent of the nation's greenhouse gas (GHG) emissions and consumes 27 percent of the energy, making fuel economy particularly important in this analysis. Table 2 shows the fuel economy and full fuel cycle greenhouse gas emissions of each automobile; the Prius has the highest MPG (miles per gallon), followed by the Civic and manual Golf and Jetta. …

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